The invention is in the field of biasing circuits for MOS integrated circuits (ICs), and relates more particularly to a method of biasing such MOS ICs to provide temperature compensation.
When the operating temperature of an MOS IC changes, various transistor parameters will also change, thus causing the bias point of the circuit to change. At a certain point, such changes may cause the circuit to operate in a less than optimum fashion, or even to cease operating in the intended manner.
Accordingly, there exists a substantial need to compensate for transistor parameter changes with changes in temperature. Although many critical circuit parameters can be made temperature independent by making them a ratio of two components, such as transistors, resistors or capacitors, of the same type, which have similar parameter variations with temperature, it is not practical or economical to use this technique for all components. As a result, circuits are typically over-designed to guarantee performance over the desired temperature range, or else an external reference component having a parameter that varies only slightly with temperature is provided, and this component is used to generate a bias condition with a minimum variation over temperature. In other cases, additional components are inserted into the circuit to compensate for parameter changes with a change in temperature.
One example of the latter form of compensation is shown in GB 2,225,913A, in which a resistor is provided in series with an MOS transistor, between its source electrode and ground, in order to make transistor operation less temperature dependent.
However, all of the prior-art temperature compensation schemes discussed above have inherent drawbacks. Thus, compensation by providing ratioed pairs of components increases the cost and complexity of the circuit, and does not fully compensate for variations with temperature, since only some components are compensated. Similarly, the use of over-design techniques also increases cost and size, and does not fully compensate for temperature variations, and the use an external reference component likewise increases cost and complexity. With reference to the specific compensation scheme of GB 2,225,913A, this technique requires that the value of the series resistor be correctly calculated based upon a precise determination of the temperature coefficients of the transistor and the resistor, so that in practice the threshold voltage of the compensated transistor is made less temperature dependent but not completely temperature independent. Furthermore, since the prior-art circuit requires the use of a resistor between the source electrode and ground, grounded-source operation is not possible. Additionally, the use of such a series resistor can limit both the gain and speed of the circuit.
Accordingly, it would be desirable to have a temperature compensation method for an MOS IC which affords excellent temperature compensation without the need for over-design, the use of redundant or ratioed components, the use of an external reference component or a series resistor in the primary circuit.